high and flat
嵬 describes a mountain or terrain that is high and flat on top, like a plateau or tableland.
嵬 combines 山 (mountain) with 鬼, which likely contributes a phonetic element. The exact historical development is uncertain, but the mountain component clearly relates to the meaning of a high, flat landform.
A mountain 山 with a demon 鬼 on top: the demon flattened the peak to make a high, flat platform for its lair.
For カイ, imagine a kite flying over the high, flat mountain: kite -> カイ, soaring above the plateau.
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